Update file session adapter to match new GenieSession

[andreeco]

This PR updates GenieSessionFileSession to work with the new GenieSession
API and storage model:

• Serialize GenieSession.Session values, Base64-encode and encrypt them
  before writing to disk.
• Decrypt, decode, and deserialize on read, and respect expires_at when
  loading.
• Implement GenieSession.persist / GenieSession.load for the new
  Session layout.
• Add a helper rotate_file_session! that calls GenieSession.rotate!
  and removes the old on-disk session file.